1. Field of the Invention
This invention relates to a lamination-type gas sensor element and a gas sensor, and more particularly to a lamination-type gas sensor element which is less susceptible to damage upon contact of an element body with a water drop, and which provides for early activation of a detection section by heating with a ceramic heater. The invention also relates to a gas sensor including the lamination-type gas sensor element.
2. Description of the Related Art
A gas sensor including a lamination-type gas sensor element is known as one kind of gas sensor capable of detecting oxygen, carbon monoxide, various hydrocarbons, nitrogen oxide (NOx), etc., in exhaust gas emitted from an internal-combustion engine or measuring the concentration thereof. The lamination-type gas sensor element for an oxygen sensor (lamination-type oxygen sensor element) has an element body obtained by laminating 1) a ceramic heater including ceramic layers and a heater element embedded in the ceramic layers, and 2) a solid electrolyte layer having a pair of electrodes sandwiching a portion thereof. The portion of the solid electrolyte layer sandwiched between the paired electrodes functions as an oxygen concentration cell and becomes a detection section for detecting the oxygen concentration. Such a lamination-type oxygen sensor element is not activated until the solid electrolyte layer reaches a predetermined high temperature. Therefore, a technique of heating the solid electrolyte layer (detection section) with a heater element from the start time of an internal-combustion engine to activate the sensor and secure early oxygen concentration detection has been proposed.
On the other hand, condensed water is usually deposited on an exhaust pipe wall under conditions where the temperature in the exhaust pipe (the temperature of the wall of the exhaust pipe) is low at the cold start time of an internal-combustion engine. Thus, if the heater element is energized for heating the detection section to a high temperature, the lamination-type gas sensor element (element body) may be damaged by thermal shock caused by water condensation. Thus, hitherto, the detection section of the gas sensor element has been protected by a metal protector having a vent hole. However, even if such a protector is used, water may enter the element through the vent hole of the protector, and damage to the element body, such as cracks in the element body, cannot be sufficiently prevented. In view of securing sufficient exhaust gas flow and sufficient gas detection performance, the vent hole cannot be made small enough to eliminate water intrusion.
To solve the above problem, a lamination-type gas sensor element has been proposed where the part of an element body which can be damaged is covered with a porous layer for suppressing direct contact of a water drop, etc., with the element body. See, for example, JP-A-2001-281210. In the gas sensor element described in JP-A-2001-281210, if a water drop is deposited on the porous layer, the water drop can be evaporated by heating the gas sensor element before it penetrates the detection section. As a result, since a large thermal shock is unlikely to reach the element body, damage to the element body can be suppressed.
3. Problems to be Solved by the Invention
However, there is a possibility that the structure described in JP-A-2001-281210 will not be able to meet the recent demand of sufficiently early activation of the detection section. Namely, the element body in JP-A-2001-281210 must be formed with a porous layer sufficiently thick to impart water resistance. However, as the porous layer is made thicker, the volume surrounding the detection section containing the porous layer increases. Consequently it takes a longer time to heat the detection section to the activation temperature using a ceramic heater. Also, each corner of the element body forming the gas sensor element is prone to damage due to thermal shock, etc., and requires sufficient damage prevention measures, but the gas sensor element described in JP-A-2001-281210 does not address this problem.